The prior art carbon arc lamps have a construction as shown in FIG. 1, comprising carbon electrodes 1 and 2, a holder 3 holding the carbon electrodes, an electromagnet 4 and a damper 5, and the lamp is adapted to automatically emit light by an electric discharge between the opposed electrodes.
A glass bulb 6 is mounted on a base 7 by a retaining spring means 8 and is removable for the replacement of the carbon electrodes. During the discharge between the carbon electrodes 1 and 2, they are combusted to produce ash, which strikes the inner surface of the base and flows along the inner surface of the glass bulb. The base is made of iron or bronze or a like alloy, and its temperature becomes as high as about 350.degree. C from the heat of the arc. The temperature of the inner surface of the glass bulb, on the other hand, is raised only to about 50.degree. C less than that of the base. Therefore, the ash is cooled when it strikes the inner surface of the glass bulb and is deposited on and dulls the glass.
Although the glass bulb may be cleaned every time the carbon electrodes are replaced, between changes it becomes gradually dirtier so that the intensity of light incident on the test sample is gradually reduced.
The prior art arc lamps thus have had the above drawback that ash is readily deposited on the glass bulb.